On a drill rig it is important to know the exact location of the drilling machine along the feed beam. This serves two purposes, firstly it indicates at what depth the drill bit is located and secondly it helps the operator when manoeuvring the drilling machine, for instance when a drill rod is added to the drill string.
Generally, a rock drill rig (see FIG. 1) comprises a movable carrier 1, a boom 2 and a feed beam 4 connected to the boom 2 via e.g. a feed beam holder 3. The feed beam and the feed beam holder are movable in relation to each other along at least part of the length of the feed beam, which permits relative movement between the feed beam and the boom. A drilling machine 5 is movably arranged on the feed beam.
It has proven advantageous to use a hydraulic cylinder or a pressure cylinder for moving the drilling machine along the feed beam as it provides a reliable and steady feeding.
A problem that arises when a pressure cylinder is used as feed engine is the placement of a measuring device.
U.S. Pat. No. 6,550,544 B1 discloses a rock drilling device, in which a pressure cylinder is used for moving the drilling machine along the feed beam. In this arrangement a feed line is routed over a sheave that is arranged at a movable end of the pressure cylinder. A separate measuring device is arranged to measure the movement of the outer end of the pressure cylinder, which corresponds to half the movement of the drilling machine. The measuring device comprises a wire that is routed over a wheel, which is connected to a sensor that registers the rotation of the wheel. This arrangement, which is used today, has two main problems. Firstly it is not always accurate enough since the wire slides around the wheel, and secondly the wire and the wheel may be subjected to mechanical efforts, which might damage the device.